1. Field of the Invention
This invention relates to an optical sensing system for industrial measurement.
2. Description of the Prior Art
From a need for a complete safeguard against explosion in a chemical plant as well as a measure against an antiground potential difference in a heavy electrical apparatus and a measure against noises etc., an optical sensing system covering the entire route from signal transmission to signal translation as an optical means has come into the limelight in place of a conventional electrical sensing system.
In a measurement by an optical means in particular, the technique of a light interferometer utilizing the shortness of an optical wavelength is currently used so that sensing can be effected at high accuracy and high sensitivity. A wavelength interferometer is known which is adapted to measure a physical quantity at a place by measuring a shift of an interference fringe which is produced by varying an optical path length of a path at that place by the physical quantity such as temperature, pressure, length etc. In the optical sensing system of this type it is easy to know a variation of a to-be-measured quantity at high accuracy by making longer an optical path at the place where measurement is effected. In order to find the absolute value of the to-be-measured quantity it is necessary to know how many light or dark bands are moved in an interference fringe. This involves a time-consuming operation.